This invention relates to a method of removing solids that are in suspension in the oil mixture derived from the liquefaction of coal.
Processes for liquefaction of coal involve the preparation of a suspension of finely ground coal in a coal derived oil or organic solvent and pumping this suspension into a high pressure reactor where it is heated to promote the dissolution of the organic components present in the coal. Hydrogen is generally added to the reactor system to increase the hydrogen to carbon ratio causing conversion of coal components into liquid fractions. The extent of conversion of the coal into oil depends on the type of coal and also on the process variables including temperature, pressure, residence time of the coal particles in the reactor, and the relative amounts of hydrogen and oil or solvent used with respect to the coal fed to the process. Conditions found effective for liquefaction of eastern bituminous coal are: temperature 700.degree. F; pressure -- 2000 psig; residence time of coal -- 30 minutes; and, the ratio of oil to coal of 3:1. Hydrogen is added in a relative amount of approximately 10-20,000 scf/ton of coal. Under these conditions, up to 90 percent of the organic constituents of the coal can be converted to liquid products.
Processes to liquefy coal which are applicable to the basic method described are not new. Such liquefaction processes were developed and used in Germany during World War II. Similar processing has been in use in South Africa for more than 20 years for making gasoline from coal. The technology of coal liquefaction has been under development by various government and private organizations with the purpose of improving the processing and thereby reducing the cost of liquid fuels derived from coal.
A part of the coal liquefaction process that is recognized to be a difficult and costly step is the removal of the solid residue particles which remain in suspension in the coal derived oil leaving the hydrogenation reactor. These residue particles comprise the unreacted organic components and the nonreactive inorganic mineral matter associated with the incoming coal. These suspended residue particles must be removed to produce oils which meet fuel specifications for ash and sulfur. The term "ash" as used herein in general refers to ash-forming mineral or other inorganic matter, as contrasted with unreacted organic matter.
Various methods can be applied for separation of solids from liquids, including sedimentation, filtration, centrifuging, and possibly others. A number of physical factors, however, make these generally used methods extremely difficult to apply without adding significantly to the equipment, labor and maintenance costs for producing liquid fuels from coal. First, the particles of suspended solids are extremely fine in size with up to 50 percent or more of the material less than about 5 to 10 microns. Sedimentation of these fines in the oil would be impractical because of the large area required to provide the needed settling time. Sedimentation therefore would require an excessive inventory of the product oil. Moreover, because the viscosity of the coal derived oils increases as the oil cools, the solids separation must be accomplished at temperatures above about 500.degree. F to assure adequate fluidity of the oil. However, the oil contains a considerable amount of components that are volatile at elevated temperature, therefore it is necessary to maintain the oil under pressure of at least about 150 psig for solids separation operations.
Solids separation methods now employed in coal liquefaction plants include filtration aand centrifugation or hydrocloning. Filtration appears to be a preferred method because it allows a higher recovery of clear oil product and is effective in removing sub-micron size particles of the residue. Although the unit operation of filtration is well established in the mineral and chemical process industries, it is usually conducted at near-ambient conditions. Filtration of the hot oils under pressure requires filter apparatus of advanced and special design not now available in the high capacity range of major interest in coal liquefaction. In addition, filtration of the hot oils is complicated by the need to use a filter aid such as diatomaceous earth to prevent the fine particles from rapidly clogging the filter screen. The filter, however, must be periodically backwashed and a fresh "pre-coat" of filter aid developed. Thus the filtration cycle includes backwashing, pre-coat preparation, filtration, cake removal, and cake drying. Of a total cycle time of about 90 minutes, the actual filtration time is only about 30 minutes. In effect, filtration of the hot oils requires a substantial product inventory and high capital investment in special pressure filters, heated liquid storage tanks, pumps, and control instrumentation.
The practice of the present invention permits the continuous and essentially complete removal of solids suspended in oils derived by coal liquefaction. The coal derived oils containing the suspended residues are contacted directly with water in a manner to cause the suspended solids to be transferred from the oil phase into the water phase. Once contained suspended in the water phase, the resulting water slurry of the residue solids can be cooled to ambient conditions of temperature and pressure and the solids separated by relatively low cost conventional methods of thickening, flocculation, filtration etc. using standard apparatus of proven design now available for operations at the full capacity of projected coal liquefaction plants. By effecting a rapid transfer of the mineral residues from the oil to the water, very little of the product oil is required to be in inventory within the process system and this greatly improves the process economy. Moreover, because the water is a relatively inexpensive agent, conventional thickeners may be employed at ambient conditions to take advantage of low cost gravity means for clarification of water for reuse and for preconcentration of the mineral residues for subsequent filtration with little or no loss of product oil in the filtration cycle.
Utilizing the methods of this invention, mineral solids contained in suspension in coal derived oils can be removed from the oil by direct contact of the hot oil with water under pressure. To enhance the overall transfer rates, it is desirable to incorporate a centrifugal action to increase the rate of coalescense of any dispersed water phase and to accelerate the migration of the mineral solids through the oil-water interface into the bulk water phase.
In conducting the process of this invention, it is desirable to establish conditions such that the oil phase has a substantially lower density than water. If there is insufficient density difference, agitated contact of the oil and water can result in the formation of an emulsion with resulting difficulty in separation of the oil and water phases.
It is known that the density of the coal derived oil phase decreases with increasing temperature. At moderate temperature, for example 150.degree. F, a coal derived oil may have a density of about 1.152 which is greater than that of water. Thus, if this coal oil and water are intimately mixed at 150.degree. F, and then allowed to settle at this temperature, two phases will form, i.e., a heavier, lower oil phase and a lighter upper water phase. Mineral residue particles originally present and suspended in the oil will tend to remain in the oil phase or adhere to the resulting oil-water interface. In this case, with the oil heavier than water, any attempt to effect the transfer of the residue particles into the bulk water phase by centrifugal action or by gravity means will cause the mineral solids to return to the heavier oil phase without achieving the desired removal. Therefore, the practice of this invention may involve mixing of the coal derived oil with water and desirably includes maintaining and controlling the temperature of the oil-water mixture so as to ensure that the coal oil phase will have a desired density which is lighter than water. It has been found that for some oils temperatures of around 500.degree. F are approximately optimum to help achieve the fluidity which assures the rapid separation of oil and water phases after mixing. Such high temperature also helps to fulfill the requirement that the coal oil be lighter, than the water, or other aqueous medium employed, for effective separation of the phases. Application of centrifugal force, as for example by hydrocyclone or centrifuge, then pulls the wetted particles through the oil-water interface into the heavier bulk water phase and efficiently and rapidly removes these particles from the oil phase.
Basically, therefore, the processes of this invention are concerned with the removal of mineral residue solids suspended in the coal derived liquefaction oils by controlling the temperature, dilution of the oil by a lighter liquid which is miscible with the oil but not miscible with the water or other aqueous medium, and/or other conditions causing the density of the oil to be substantially lighter than water and accelerating the oil to cause rapid transfer of the mineral solids from the oil into the bulk water phase.
The coal extract liquid at ambient temperature may be heavier than water, but if this oil is diluted by first adding a suitable organic solvent, such as benzene, the resulting mixture can be made lighter than water at ambient temperature. Subsequent application of centrifugal action enables mineral residue solids to be rapidly and efficiently removed from the coal oil. It has been found that an effective amount of added solvent to accomplish the stated purpose is of the order of a volume ratio of 1:1 solvent to coal oil.
Separation may also be enhanced by control of the chemistry of the system, for example the pH, which may induce flocculation and thereby enhance the effect of centrifugal action in causing the suspended particles to migrate from the oil to the water phase. In addition, certain chemical agents may be added to affect and control the viscosity of either the oil or the water phase to accelerate particle transfer or to cause the particles to be more rapidly wetted by water or to reduce and minimize the quantity of oil phase occluded on the mineral solids' surface and present after transfer of the particles from the oil to the water.
The relative quantities of oil-solvent and water can be controlled as well as the temperature and chemistry of the system to assure optimum process rates and recovery.